High-pressure electric discharge tube



Aug. 30, 1960 T. HOLMES HIGH-PRESSURE ELECTRIC DISCHARGE TUBE Filed July 21, 1958 INVENTOR. 7'. HOLMES wgu;

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United States Patent O HIGH-PRESSURE ELECTRIC DISCHARGE TUBE Thomas Holmes, Eindhoven, Netherlands, assignor to North American Philips Company, Inc., New York, N.Y., a corporation of Delaware;

Filed July 21, 1958, Ser. No. 749,993 Claims priority, application Netherlands Aug. 10, 1957 3 Claims. (Cl. 313-209) In U.S. Patent application Serial No. 640,940, filed February 18, 1957, now U.S. Patent 2,887,603, a highpressure electric discharge tube is described comprising electrodes the relative spacing of which is small as compared with the tube diameter, one of the electrodes, in a vertical arrangement preferably the lower electrode, being provided with a conical cavity, the apex of which merges into a cylindrical bore, while the discharge current is set at a value exceeding that at which the discharge involving a focal spot passes into a discharge without `focal spot.

When the cylindrical bore of the electrode opens into a space the sectional area of which exceeds that of the bore, provision being made in this space of a material having a higher electron emision than the material of the hollow electrode body, it has been found that the emissivity of this electrode may cease or at least fall off materially before the supply of emissive material is spent.

It is an object of the present invention to obviate this disadvantage.

According to the present invention, provision is made in the said space of a member which is in contact with the wall of the cylindrical bore and with the emissive material. This member ensures a constant migration of the emissive material to the cylindrical bore.

This member may be a helically coiled metal wire. If desired, it may be a double helix comprising two layers.

In order that the invention may readily be carried into eiect, two examples thereof will now be described in detail with reference to the accompanying drawings.

In the gures, reference numeral 1 denotes a substantially spherical quartz discharge tube having an inside diameter of about 20 mms; it is iilled with xenon at a pressure of about atms. at room temperature.

Tungsten electrodes 20 and 30 are provided in line with each other in the vertical axis of the discharge tube with a relative spacing of about 2 mms.

The electrode ends facing each other are provided with conical cavities 21 and 31 which at their apices merge into cylindrical bores 22 and 32.

Tungsten current supply leads 23 and 33 are connected to the electrodes by sleeves 24 and 34 respectively each comprising two layers of helically coiled tungsten wire with engaging turns.

In the spaces bounded by each electrode, a current supply lead and a sleeve, provision is made of an emissive material. A member which is in contact with the cylindrical bore is in contact with the emissive material also.

This member and the emissive material are not shown in Fig. 1.

In Figures 2 and 3 the lower electrode is drawn to an enlarged scale for the sake of clarity.

In the embodiment shown in Fig. 2 the said member consists of a helically coiled tungsten wire 37 having a diameter of about microns one end of which is immersed in the supply of emissive material 38 while the other end engages the wall of the cylindrical bore 32. The upper turns of the helical wire may have a larger pitch than the lower turns.

This member promotes the migration of the emissive material to the cylindrical bore so that the emissivity of the electrode is less likely to cease prematurely than without the provision of such a member.

In the embodiment shown in Fig. 3, the helix 37 is provided -by winding back the wire with a second layer 39 which extends to the lower end -face of the electrode body 30. This double helix has the advantage of holding more emissive material than a single helix.

This emissive material may be a mixture of barium oxide and thorium oxide.

Through an impedance (not shown) the tubev was operated with alternating current of about 9' amps. with a tube voltage of about 18 volts. The discharge was quiet. At a current strength exceeding about 2 amps. there were no focal spots. The life was about 500 operating hours, the decrease of the luminous flux being about 25%.

The diameters of the electrode 20 and the current supply lead 23 were about 2 mms., the diameters of the electrode 30 and the current supply lead 33 about 1.4 mms., the diameter of the bore 22 about 0.8 mm. and that of the bore 32 about 0.5 mm. The conical cavities 21 and 31 had apical angles of about 120. The axial spacing between each electrode 'and its associated current supply lead was about 2 mms. 'Ihe length of the sleeve 24, which consisted of a wire having a diameter of about 0.7 mm., was about 6 mms. and that of the sleeve 34, which consisted of a wire having a diameter of about 0.5 mm., was about 5.1 mms.

What is claimed is:

1. A high-pressure electric discharge tube adapted to operate without a focal spot comprising a light-transmissible envelope, an inert gas filling said envelope at a pressure of several atmospheres, a pair of axially aligned spaced conductive electrodes within said envelope, the

spacing of the electrodes being small compared with the ldiameter of the envelope, at least one of said electrodes having a conical depression the base of which faces the other electrode and the apex of which lies within and on the axis of said electrode, said electrode also having, an axial cylindrical recess extending from the apex of said conical depression into a chamber in the electrode having la sectional area exceeding that of the recess and which is at least partially lled with a material having a higher electron emission than the material of said electrode, and a helically coiled metal wire in contact with the wall of said recess and the emissive material to facilitate constant migration of emissive material into said recess.

2. A lamp as claimed in claim 1 in which the wire is wound in a double helix having two layers.

3. A lamp as claimed in claim 1 in which the emissive material is a mixture of barium oxide and thorium oxide.

References Cited in the file of this patent UNITED STATES PATENTS 2,304,412 Kern Dec. 8, 1942 2,821,647 Rigden Ian. 28, 1958 2,887,603 Haidinger May 19, 1959 

